1. Field of the Invention
The invention concerns a frequency synthesizer stage designed to generate very high frequencies by adding high frequency steps to a frequency, derived, for example, from preceding stages of a synthesizer and itself comprising smaller frequency steps.
2. Description of the Prior Art
The process is already known, from French Pat. No. 73 17650 filed by the Applicant on the 16th May 1973 for "A variable frequency electric signals generator suitable for operating in a wide range", for realizing such a frequency summation by means of two voltage controlled oscillators each controlled by a phase locked loop, the first loop dividing the frequency Fo+.DELTA. derived from preceding stages (.DELTA. being the sum of the low steps) by a variable integer N by means of a programmable divider and adding to it a standard frequency P which represents the high step, while the second loop multiplies the intermediate frequency F.sub.A supplied by the first loop by the same factor N so as to give an output frequency F.sub.S =Fo+.DELTA.+NP.
The drawback of this mode of generating of large steps is that the frequency F.sub.A and, consequently, the phase noise which affects the output frequency, are multiplied by a factor N which is all the larger because the number of frequency steps required is higher.
According to U.S. Pat. No. 4,458,329 filed by the Applicant on 2nd Dec. 1981 for a Frequency synthesizer including a fractional multiplier, the dividing factor N is replaced by a factor NR+M, R and M being integers, and the multiplication factor N by a factor R times smaller than N+M/R. The phase noise which affects F.sub.S is thus reduced in a ratio R without the number of large frequency steps being reduced.
However, this process is limited in practice to the synthesis of 40 or 50 large steps with a degradation of spectral purity in the order of 20 dB, corresponding to R=4 or 5.